The default mode network (DMN) is a set of brain regions consistently activated at rest and deactivated during task performance. DMN activity is abnormal in many neurological and psychiatric disorders, including major depressive disorder (MDD) and drug addiction. The two main parts of the DMN are the medial prefrontal cortex (mPFC) and posteromedial cortex (PMC), two areas that are spatially distant and have distinct canonical functions. While there is some evidence of direct connections between these general cortical areas, little is known about their specifics and how they link the network together as a whole. I hypothesize that the DMN is linked together by specific hub subareas that contain converging connections from multiple DMN regions, and the cingulum bundle and the internal capsule are the major connecting white matter bundles for this network. Thus, the goal of this proposal is to delineate how anatomical connections and pathways between mPFC and PMC allow them to operate as a network. Combining traditional anatomical techniques with diffusion magnetic resonance imaging (dMRI), I will determine the connections that underlie the DMN. Defining these specific DMN connections will establish the circuitry subserving neuroimaging results. This basic knowledge is fundamental and is the first step in understanding the changes in the DMN in disease. Functional connectivity within the DMN may result from direct anatomical links, indirect ones, or some combination of both. The first overall aim of this grant i to delineate the direct and indirect connections between subregions of the mPFC and PMC to find potential zones of converging connections. Furthermore, dMRI has identified psychiatric disorders, including MDD and addiction, with specific abnormalities within white matter pathways that likely link DMN structures. These abnormalities likely reflect disruption of specific connections. However, the fibers traveling through any given location within a white matter bundle remain unknown. The second and third aims are to establish white matter pathways that connect DMN structures using tracing and dMRI methods. This will link dMRI and anatomical tract-tracing, testing the validity of dMRI and providing a guide for how to interpret its results.